Spectral purity filters have long been employed in UV (ultra-violet) lithography systems that are designed for integrated circuit (IC) fabrication. In certain UV lithography systems (which, as the term is employed herein, also includes deep UV or extreme UV lithography systems), one or more spectral purity filters may be employed to filter out out-of-band radiation (i.e., light that is outside of the range of wavelengths of interest).
Although the examples herein discuss UV light, it should be understood that a spectral purity filter may be designed to filter out light of any wavelength, ranging from UV to infrared (IR) for example. In a typical example, a spectral purity filter may be created using a thin-film filter made of a material that permits light of certain wavelengths to pass through while blocking light of other wavelengths. Example materials that may be employed for such spectral purity filters (for 13.5 nm central wavelength) include, for example, Zirconium (Zr) and Silicon (Si).
One of the more important considerations in the design of a spectral purity filter is transmission efficiency. All things being equal, a spectral purity filter that permits more of the in-band radiation (i.e., light that is in the range of wavelengths of interest) to pass through is more desirable than a spectral purity filter that blocks more of the in-band radiation.
Due to the thinness of the thin film material with which spectral purity filters are fabricated, mechanical support is also a critical design consideration. For horizontally disposed spectral purity filters, for example, it has been estimated that the limited inherent strength of the spectral purity filter material limits the filter size to a few (such as 10-12) square millimeters.
To construct a larger spectral purity filter, mechanical support structures have been proposed. In a prior art configuration, a spectral purity filter may be disposed in a frame that is made of a suitable supporting material, such as metal or some form of composite material. The frame provides mechanical strength around the periphery of the spectral purity filter.
Further, a mesh may be attached to the frame such that the thin film material of the spectral purity filter may be supported by the mesh material. The openings in the mesh would allow light that is within the range of wavelengths of interest (i.e., in-band radiation) to pass through. By using a mesh to support the thin film material, it is possible to create a larger spectral purity filter out of an inherently fragile thin film.
However, the presence of the mesh material reduces the transmission efficiency of the spectral purity filter assembly. As discussed, in-band radiation may pass through the openings in the mesh of a spectral purity filter that is mesh-supported. However, a non-trivial portion of the in-band radiation is blocked by the mesh material itself. This blockage reduces transmission efficiency and is thus undesirable.